Ventilation Strategy for Condensation Management (Part 1)

Ventilation Strategy for Condensation Management.

A functional extractor fan with common wiring arrangements fails to prevent condensation in this concrete block.

I have previously outlined my views as to whether or not residents are responsible for condensation damp and we believe that they very rarely are. Residents cause humidity but generally speaking it is buildings that cause condensation and so it is critically important to understand ventilation strategy. That statement will make uncomfortable reading for those landlords relying on spurious claims that condensation damp is an occupancy issue. A potential client called me just this week and told me that her landlord, a large local authority in London, had refused to deal with dampness in her flat because it was proven to be condensation and therefore it was her problem and not theirs. Quite an outrageous position to adopt and one I’m sure will end up costing them considerably when they land in court. I’m sure that the flawed logic behind their position is that the occupant created the humidity and they simply have to manage that humidity by opening their windows and using their heating; advice that is clearly contradictory because it is akin to puncturing your petrol tank just after you’ve filled her up. You’d really like to keep that expensive petrol in the tank but you’ve been advised that the tank is safer with less petrol and more air so puncturing the tank works a treat. I hope my analogy illustrates the that fact that opening your windows in the middle of winter to manage condensation is nuts, and if anyone else ever gives you that advice you should tell them so.

I gave a talk on dealing with damp in old and historic buildings at the RICS conference in Loughborough this week and there was some focus on ventilation strategies because by far the number one damp problem that we see in old solid walled properties is condensation damp. You could argue that this is simply because we always house really bad tenants in old solid walled properties, irresponsible tenants who are incapable of managing their own humidity but of course the idea is ridiculous; could it simply be that old solid walled properties are thermally inferior to modern properties? It’s a rhetorical question because we rarely encounter ‘occupancy’ related condensation damp and where we do then it is generally caused by over-occupancy and is a housing management, rather than a building technical issue. Please review The Condensation Trap

Why Opening Windows is a Very Bad Idea

Air temperature versus Maximum Moisture Content

We know that old solid walled properties are thermally inefficient and suffer from thermal bridging. They are prone to suffering condensation damp because thermal bridging causes cold internal wall surfaces that are often below dew point temperature. Therefore warmth and heat retention is an equal consideration to achieving air changes within the property. We know that warm air holds more moisture than cool air so an increase in ambient temperature will immediately reduce the internal humidity levels. Lets say that we have a room temperature of 20°C and an internal humidity of 80 percent, if we turn the heating up to 25°C then the internal humidity immediately reduces to circa 60 percent thereby reducing the risk for condensation. The secondary benefit is that this increase in ambient temperature will raise internal wall surface temperatures and may well raise them above dew point temperature, which is another substantial risk reduction, and bear in mind we’ve not even thought about air changes yet.

If we adopt the more common approach of opening windows to achieve air changes something quite different happens… Again imagine that we have a property with a room temperature of 20°C and an internal humidity of 80 percent. It’s chilly outside with external temperatures of only 3°C but we open the window anyway and this allows cold air to immediately enter the property, reducing the ambient temperature and further chilling the building fabric possibly below dew point temperature; this in itself is an increased risk for condensation. Air at 20°C can hold 15 grams of water vapour per kilogram of air but since the ambient temperature has now reduced, lets say to 5°C, then the air can now only hold 5 grams of water vapour per kilogram of air. So what happens to the missing 10 grams of water vapour? Well quite simply it is immediately given up as transient condensation within the property, the cooling effect of incoming air is so quick that humidity does not have time to exit the building. Transient condensation is what frequently occurs in bathrooms when occupants open windows after a shower and the immediate effect of transient condensation convinces everybody that the steam exited the window but in fact they have been fooled. If it is cold outside and you open a window it will cause transient condensation and of course also results in very high levels of expensive heat loss.

Why We Need an Effective Ventilation Strategy

In simple terms, uncontrolled ventilation (open windows or air bricks) is a very bad idea whilst controlled ventilation is a very good idea. Efficient ventilation and air changes are important in a property but it should not be achieved at the expense of substantial or acute heat loss within the property. There needs to be a balance between achieving the required air changes within the property and a parallel objective to manage or spread heat losses over a longer time period so as to significantly reduce the risk for transient condensation. An effective ventilation strategy will account for heat losses within the property as well as air changes. In part two I’ll discuss the types of ventilation available, and how best to use the more commonly available methods of ventilation.

2 Comments

If there is an issue in buildings requiring dispassionate research and analysis in the modern world, it’s condensation. This and Part 2 of the article are interesting lines of thought but cover only a small part of the ground.

Occupancy issues cannot simply be written off as an irrelevance. I am directly responsible for a dozen tenanted properties – all older and with mainly solid walls, some listed. As a Surveyor, I have dealt with condensation issues in scores of properties. The picture is crystal clear: the same older property with the same potential for condensation issues can experience no problems for 5 or 6 occupiers in succession but be covered in black mould within a couple of months of the next resident moving in. This can only be explained by occupancy practices.

I can quite see that mechanical ventilation with heat exchange may have a part to play in reducing the potential for a problem and we have a project pending in which I may well try just that. So what follows is not negating the two published articles but seeking to add to them.

In my experience, there are two key considerations: minimising vapour emissions (and getting rid of concentrated moist air as quickly as possible); and avoiding fluctuations in internal temperature – as between one time in the day and another, as between one room and another, and within a given room. It is air cooling below its dew-point that is the problem: so either reduce the moisture content; or avoid it cooling sufficiently to reach that dew-point.

Anything that produces unnecessary moisture in the home needs to be minimised: clothes drying indoors; LPG heaters; baths filled first with hot water and then cooled with cold; over-hot showers; major boiling activities using uncovered pans. Over and again, I have found that attention to just one of those things eliminates the problem.

Where concentrated steam is produced – get rid of it. Mechanical extraction and opening windows in specific circumstances (and hindering warm moist air from moving from one room to a colder neighbouring one) do have a part to play.

But also key are heating patterns. If some rooms are heated more than others, warm moist air will move into cooler rooms and condense there. And if the heating is peaked at times of moisture production (when getting up in the morning and after work in the evening), allowing troughs in temperature between those peaks, my experience is that the risk increases dramatically. Again, from experience, a one degree reduction in the thermostat setting coupled with running the heating for an hour or so longer in each session (better still, 24 hrs a day) can make the difference which completely eliminates an apparently intractable problem.

Finally – keep that air moving. This brings us back to mechanical ventilation but if that is not an option, stratagems to minimise the risk of air stagnating next to cold surfaces can pay big dividends – fans, floor level blow heaters, ventilation ducts in the top and bottom of wardrobes, moving furniture a little away from cold walls – all those things do help and are within the control of residents (whether owners or tenants) without the need for disruptive work or capital expense.

The steps mentioned here may not address all circumstances but shouldn’t be ignored in the rush for a ‘technical’ solution.

Thank you very much for you detailed post Nigel, some of which I agree with, most of which I do not. I think it’s important that I respond to your comments in detail…

Firstly your response is slightly of out context since the two part blog deals purely with ventilation strategy. It would have helped to contextualize your own response if you’d posted against this blog, which deals with the broader issues of occupancy, heating etc.

You state that; “If there is an issue in buildings requiring dispassionate research and analysis in the modern world, it’s condensation.” I disagree, condensation is extremely easy to diagnose and understand but only when surveyors are prepared to attend site with the correct diagnostic equipment and can apply an holistic investigation process. We attend our investigations with circa £15k’s worth of diagnostic equipment and I wonder what diagnostic equipment you attend site with? I recently gave a talk for the RICS at their East Midlands CPD conference on damp in historic buildings and as part of that talk showed two slides, one containing a picture of the diagnostic equipment we take to site and another showing the absolute bare essentials. When I asked the audience of about 160 people how many carried even the bare essentials only two surveyors put up their hand. Yet despite these statistics surveyors like to talk about their ‘experience’ of damp buildings when experience is quite often confined to visual inspection.

You also state that; “This and Part 2 of the article are interesting lines of thought but cover only a small part of the ground.”

Agreed, but even the blog title makes it very clear that the blogs are dealing with a limited area, ventilation strategy.

You state that “Occupancy issues cannot simply be written off as an irrelevance.” At no point in these blogs or in any other blogs do I say that occupancy issues are an irrelevance. However, I make a very clear point that there is no test to prove an occupancy issue and you can only come to this conclusion after eliminating all other factors. I found it slightly contradictory that you think there should not be a “rush” to a technical solution because actually isn’t the reverse true? The technical solutions can not be rushed but for those surveyors wanting an easy solution in the absence of any scientific evidence then there has been a rush to blame occupancy issues for many years. Whilst not an “irrelevance” we are firmly of the opinion that occupancy issues are extremely rare, an opinion formed on the back of hundreds of detailed damp investigations using the full range of diagnostic equipment that has allowed us to provide objective proven conclusions.

I find this point particularly interesting and worrying in equal measure, you state that; “The picture is crystal clear: the same older property with the same potential for condensation issues can experience no problems for 5 or 6 occupiers in succession but be covered in black mould within a couple of months of the next resident moving in. This can only be explained by occupancy practices.”

You have taken anecdotal evidence and used it to draw a conclusion that simply isn’t there and here’s why… Lets assume that the property is a small two bed solid walled Victorian terraced. For many years occupied by only one person with no issues of condensation damp or mould. The property is sold to a couple with one child who within weeks of moving in start to experience issues with condensation and mould. They’re using their heating no different to the last occupant but clearly there is more cooking taking place, more showers and of course more moisture being exhaled. However, despite the problems the occupants are not actually behaving any more unreasonably than the last occupant. Your default position is that these new occupants must now be responsible for the damp problems but this is rarely the case. The property would not be considered as being over-occupied but the increase in occupants has raised internal relative humidity enough to highlight the buildings thermal bridging issues. As I constantly say, occupants cause humidity, but this is not condensation! Should a two bed property be technically capable of dealing with the normal humidity generated by three occupants? Absolutely! We have encountered over-occupancy and a technical solution is not the answer to what is a housing management issue. Of course we assess occupancy issues as part of the holistic investigation process but unreasonable behavior is rare. We recently had a lady constantly washing clothes and using an unvented tumble dryer every day that was causing RH levels of circa 90% within the property. Clearly this is an occupancy issue but dryers are now generally condensing units or vented to outside and we rarely, if ever, see calor gas heaters being used indoors, which was a problem many years ago. Problems caused by cooking without lids or drying washing indoors are overstated and we would argue that tenants have a right to dry washing indoors without being told that their behaviour is unreasonable. Properties increasingly have little or no outdoor space and many occupants have little choice but to dry washing indoors, which means that the building should be designed to facilitate this need.

I think we are in broad agreement on rapid extraction of moisture during key moisture generation times, particularly showering and bathing. However rather than worrying about air cooling below it’s dew point; from a surveyors perspective it is more useful to concern yourself with surface temperatures that may be below dew point; an issue that can easily be checked with surface or laser thermometers.

The point you make here is contrary to our own experience, you state that “Anything that produces unnecessary moisture in the home needs to be minimised: clothes drying indoors; LPG heaters; baths filled first with hot water and then cooled with cold; over-hot showers; major boiling activities using uncovered pans. Over and again, I have found that attention to just one of those things eliminates the problem.” It is the last point that we find highly unusual because our experience is that one simple action rarely eliminates condensation damp, rather it is a complex interplay of factors and you approach potential solutions in a pragmatic and cost effective manner, starting first with the cheapest and simplest solution and moving on to more expensive options if the cheaper options fail.

You talk about “hindering warm moist air from moving from one room to a colder neighbouring one” but the idea and the theory make no sense whatoseover. What about cold moist air? Are high levels of humidity prevalent only in warm air? Clearly they are not and we see high humidity across a range of properties irrespective of air temperature. Moreover, it is rare to move from room to room during the survey process and see significant temperature differentials, only unheated areas such as cellars or the roof space tend to be significantly different so I am unsure what point you are trying to make here?

You talk about heating patterns and further state that “warm moist air will move into cooler rooms and condense there.” But this is not necessarily the case at all. Surface temperatures need to be below dew point temperature for moisture to condense out on them. A cooler room may have lower surface temperatures but this does not automatically mean that they are below dew point temperature. We never make such subjective claims we simply test wall temperatures and note the results in our report. Again, we rarely see any significant difference in ambient room temperatures across the property, rather we tend to see temperatures below dew point at wall bases (because heat rises), on external perimeter walls, on cold ceiling filets, where rooms are in roof space, cold tile surfaces, white bathroom ceramics etc. Temperature differentials between rooms rarely play a part but indoor/outdoor differentials do.

I’m half with you on heating, we generally recommend that heating is run on the stat rather than a timer to ensure building fabric remains warm throughout the day. Furthermore the heating system should be fully controllable to allow for temperature adjustment in each room.

I’m struggling to understand even the theory or relevance of this point… “And if the heating is peaked at times of moisture production (when getting up in the morning and after work in the evening), allowing troughs in temperature between those peaks” Heating isn’t actually peaked at times of moisture production and there is no proven link that is the case. Think this through logically… relative humidity generally peaks overnight when occupants are all at home sleeping, at the same time heating levels are generally reduced, thereby facilitating even higher RH levels. As the heating kicks in when occupants rise then the air temperature increases and consequently there is a reduction in relative humidity.

Finally, you say – “keep that air moving”. And talk about “stratagems to minimise the risk of air stagnating next to cold surfaces” Both points make no sense from a scientific or technical viewpoint. The movement of air in itself does nothing to add or detract from the condensation damp issues, similarly air ‘stagnating’ next to a wall means nothing. It overcomplicates the issue and provides no useful advisory or technical guidance. The idea that stagnating (still) air is a problem next to a cold surface makes little sense and again over complicates what is a simple issue. It is irrelevant whether air with a humidity of 80% is moving or static, for either case if walls are below dew point temperature then moisture will condense out irrespective of airflow. Is this just an over complicated way of saying that we need air changes in the property to reduce humidity levels? If so, then consider this.. when we carry out an internal condensation risk assessment we check both internal and external humidity’s. Would you believe that we’ve recorded higher external RH than internal? Would we then want to change internal air for even wetter external air? How would this help the condensation damp issue on that given day? I make tthe point to dispel any common assumptions that external air is always drier. In the condensation season during wet, cold or foggy weather high external humidity is not unusual. Again only a full scientific investigation will allow you to provide the bespoke information and advice required for each situation you encounter. One thing is for sure, many surveyors continue to adopt the default position that where condensation is evident then this is an occupancy issue. This is a convenient misrepresentation of the facts that suits the majority of surveyors who fail to even carry the bare essential equipment required to carry out a competent damp investigation.